After the Battle of New York, Diaz was called with his co-workers, Adam Cross and Frank Whalen, and other fire crews to douse fires in the area and to insure civilians were safe. During their time there, Diaz found a Chitauri Helmet and took it with him as a souvenir.
Diaz kept it in a display case at the fire station in Wrigley, but contracted an alien disease by taking it out of the case and polishing it months later. Phil Coulson's S.H.I.E.L.D. team tracked him down, assuming he was the killer of Cross. However, when Coulson learned the truth, he sat down with Diaz to reassure him about death and finality before exiting the fire station and leaving Diaz to his unfortunate fate.